Google provides no API to measure extension activation. However, some extensions can open web pages when in use.
We can track this use through Browsing.
Many extensions are represented by a code. With GAT you can map this code to a meaningful name so that is how it appears in reports.
In the screenshot above in the “Site URL,” we can see long ‘code’.
You can also tag these pages as ‘Extensions’ for easy filtering and reporting.
Once a tag is applied, you can filter for this app in the Browsing Explorer with the tag marked ‘Extension’ to see only extension use and the time extensions were used and a number of start events for extensions.